This invention relates to semiconductor lasers and more particularly to multiple wavelength p-n junction lasers with separated waveguides.
In recent years, the technique of impurity induced disordering (IID) has been developed as a means for crafting semiconductor structures. This technique may be defined as a process of an enhanced rate of interdiffusion of ordered elemental constituents as initially formed in consecutively deposited layers of semiconductor compounds or alloys as a result of the introduction, i.e. diffusion, of an impurity into the layers. The utility of IID, as discussed in K. Meehan et al., "Disorder of an Al.sub.x Ga.sub.1-x As-GaAs Superlattice by Donor Diffusion", Applied Physics Letters, Vol. 45(5), pp. 549-551, Sept. 1, 1984 and in U.S. Pat. No. 4,639,275, has been demonstrated in the fabrication of buried heterostructure lasers, as per the article of R. L. Thornton et al. entitled "Highly Efficient, Long Lived AlGaAs Lasers Fabricated by Silicon Impurity Induced Disordering", Applied Physics Letters, Vol. 49(3), pp. 133-134, July 21, 1986.
An alternate technique of disordering is possible through the implantation of elements acting as shallow or deep level impurities, such as Se, Ge, Mg, Sn, O, S, B, F, Be, Te, Si, Mn, Zn, Cd, Sn, or Cr, followed by a high temperature anneal at temperatures optimum to each particular impurity, e.g. 500.degree. to 900.degree. C. depending upon the particular type of impurity and best performed in the presence of excess column V element, e.g. As. It has also even been shown to be possible to disorder by implantation of III-V elements, such as Al or As. It has also been further shown possible to use a wide variety of elements to bring about disordering through implantation and annealing. For example, the inert element, Kr, has been shown to induce disordering. In the case of an impurity implant followed by an anneal, the anneal temperatures are relatively higher than diffusion temperatures, e.g. above 800.degree. C.
Multiple-wavelength diode lasers are presently being developed as advanced sources for optical communications, optical disk systems, and other information processing applications. The approaches to monolithic fabrication of such sources have thus far utilized lasers built on either a single active layer or multiple active layers of different alloy composition and hence different bandgap energy.
One approach has been the integration of several lasers operating at different wavelengths into a single chip by utilizing DFB gratings with different periods for frequency selective optical feedback in each laser. A single active layer has also been used with a grating of a single period to operate two lasers at different wavelengths by changing the stripe width to vary the index of refraction. Varying the stripe width in Fabry-Perot lasers will control the cavity loss and thereby obtain lasing on either the first or second quantized energy levels of a single quantum well active layer.
Active layers of different composition were first used to make LEDs and then lasers. In this approach, two active layers separated by a common clad layer are uniformly grown and then separately accessed by selectively removing one active layer in the post-growth processing. Etch-and-regrowth techniques have also been used to produce lateral regions of different alloy composition on which to fabricate two separate diode lasers emitting different wavelengths. Simultaneous lasing in four distinct wavelengths has been achieved by incorporating four active layers of different composition into the same optical waveguide. Recently, a graded growth technique has been developed for MBE to provide lateral variation in lasing wavelength. These past approaches are described in J. E. Epler, D. W. Treat, S. E. Nelson, and T. L. Paoli, "Multiple-Wavelength Diode Laser SuperArray", IEEE J. of Quantum Electronics, Vol. 26, No. 4, pp. 663-668, April 1990.
A p-n junction semiconductor can easily be used for a single waveguide, single wavelength laser. It is difficult, however, in a semiconductor structure with two separated waveguides to inject carriers into both waveguides with a planar p-n junction, since the layer or layers between the active waveguides will prevent injection of positive carriers into one waveguide and negative carriers into the other. Separate waveguides then require separate p-n junctions that must be separately addressed by removing layers in order to expose one access to one junction, as described in S. Sakai, T. Aoki, and M. Umeno, "InGaAsP/InP dual wavelength lasers," Electron. Lett., vol. 18, No. 1, pp. 17-18, January 1982.
It is an object of this invention, therefore, to provide a multiple wavelength semiconductor laser that is simple to fabricate.
It is another object of this invention to provide a p-n junction semiconductor laser with separate waveguides emitting at different wavelengths that are individually addressable.